1. Field of the Invention
The present invention relates to a digital circuit for the extraction of the phase and envelope signals of a single sideband signal.
2. Description of the Prior Art
A known way of obtaining a modulated signal according to the principle of single sideband (hereinafter called SSB) modulation is to separate the SSB signal into its two components, namely the phase and envelope components, in order to apply them by means of two distinct channels to a first input and a second input of the modulation stage of a power stage of an emitter.
Devices such as these are described, for example, in the U.S. Pat. No. 2,666,133, filed on behalf of Mr. KHAN, or again in the European patent application No. 0 202 985, published on Nov. 26, 1986. Other devices working according to this same principle are also known from the published work by Mr. Leonard KHAN, "Single Sideband Transmission by Envelope Elimination and Restoration" in the IRE journal Processing, Vol. 40, No. 7, July 1950, or again the published work by the same author, "Compatible Single Sideband", also published in the I.R.E. journal in July 1961.
The main drawback of these devices is that, notably in shortwave broadcasting applications, they require a highly stable and hence very costly quartz filter to carry out the filtering of one of the two sidebands which are at a distance of only 300 Hz from each other about a carrier frequency located in the region of some MHz. Although entirely digital passband filters have been envisaged, these filters prove to be inapplicable because they are very difficult to make. The recursive filters have their performance characteristics limited by the centers of their transfer function which possess a modulus very close to the unit circle and lead to instabilities. By contrast, the use of recursive filters, which by their very essence guarantee stability, requires computation times that are all the greater as the number of the points at which they are computed is high. For example, to obtain an attenuation of 40 dB of the lower sideband it is necessary, with this approach, to use a finite pulse response filter determined by 4000 points. This is unfortunately incompatible with the transmission times of the receivers. Nor can approaches using Hilbert filters be envisaged for it is very difficult to obtain a gain of 1 with a 90.degree. phase-shift. Furthermore, to obtain attenuation values, for a sideband, of less than 40 dB with phase variations around -90.degree. that do not exceed 1.14, these relationships in theory require sampling frequencies of some megahertz, which it may not be possible to achieve in practice.